Light-emitting device and manufacturing method thereof

ABSTRACT

The present disclosure provides a light-emitting device and manufacturing method thereof. The light-emitting device comprises: a metal connecting structure; a barrier layer on the metal connecting structure, the barrier layer comprising a first metal multilayer on the metal connecting structure and a second metal multilayer on the first metal multilayer; a metal reflective layer on the barrier layer; and a light-emitting stack electrically coupled to the metal reflective layer, wherein the first metal multilayer comprises a first metal layer comprising a first metal material and a second metal layer comprising a second metal material, and the second metal multilayer comprises a third metal layer comprising a third metal material and a fourth metal layer comprising a fourth metal material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 15/581,909,filed on Apr. 28, 2017 which is a Continuation of application Ser. No.15/078,355, filed on Mar. 23, 2016 now patented as U.S. Pat. No.9,666,780 on May 30, 2017, which is a Divisional of application Ser. No.14/556,032, filed on Nov. 28, 2014 now patented as U.S. Pat. No.9,331,249 on May 3, 2016, for which priority is claimed under 35 U.S.C.§ 120; and this application claims priority of Application No. 102143141filed in Taiwan, R.O.C. on Nov. 28, 2013, the entire contents of all ofwhich are hereby incorporated by reference.

FIELD OF DISCLOSURE

The present disclosure relates to a light-emitting device andmanufacturing method thereof, in particular to a light-emitting devicehaving a barrier layer comprising a metal multilayer and manufacturingmethod thereof.

BACKGROUND OF THE DISCLOSURE

FIG. 1 shows a structure of a conventional light-emitting diode. Thelight-emitting diode comprises a permanent substrate 109, alight-emitting stack 102, a metal reflective layer 106, a barrier layer107, and a metal connecting structure 108 disposed on the permanentsubstrate 109. In addition, a first electrode 110E1 and an extendingelectrode 110E1′ are disposed on the light-emitting stack 102, and asecond electrode 110E2 is disposed on the permanent substrate 109 forcurrent conduction.

The metal reflective layer 106 is for reflecting light emitted by thelight-emitting stack 102. The metal connecting structure 108 is forconnecting the permanent substrate 109 and the barrier layer 107 byconnecting two layers of materials to form an alloy. The barrier layer107 is disposed between the metal reflective layer 106 and the metalconnecting structure 108 to prevent metal diffusion between the metalreflective layer 106 and the metal connecting structure 108. However,the formation of the metal connecting structure 108 is generallyperformed at a high temperature, for example, a temperature higher than300° C., and the composition of the metal connecting structure 108 areusually different from composition of the metal reflective layer 106. Inother words, composition of the metal connecting structure 108 andcomposition of the metal reflective layer 106 do not comprise the samemetal element. For example, silver (Ag) is used for the conventionalmetal reflective layer 106, and an alloy with zinc (Zn) as the maincomponent, such as an alloy of zinc (Zn) and aluminum (Al), is used forthe metal connecting structure 108 to facilitate a high-temperaturebonding. When the metal reflective layer 106 and the metal connectingstructure 108 do not comprise the same metal element, a thin barrierlayer (less than 100 nm) for the conventional barrier layer 107 is ableto prevent metal diffusion between the metal reflective layer 106 andthe metal connecting structure 108.

However, with the development of applications of the light-emittingdiode, the requirement for performance is gradually increased. Forexample, when a light-emitting diode is used in the automotive field,for the safety concern, requirement for the reliability for alight-emitting diode used in the automotive field is higher than thatfor a light-emitting diode used in the display field. Therefore, amaterial with better stability is required for the metal reflectivelayer. In addition, compared with silver which tends to haveelectro-migration, using other metal material as the metal reflectivelayer 106 has advantages. Furthermore, the low-temperature bonding ofthe metal connecting structure 108 becomes a trend, the materialselection of the metal connecting structure 108 needs to be diversified.When the metal reflective layer 106 and the metal connecting structure108 comprise the same metal element, because there is same metal elementexisting on both sides of the barrier layer 107, other elements in thealloy of the metal connecting structure 108 are particularly easy to becombined with the same metal element on both sides of the barrier layer107. Accordingly, the design of thin barrier layer is unable toeffectively prevent metal diffusion between the metal reflective layer106 and the metal connecting structure 108. After the high-temperatureprocessing steps in the manufacturing processes, the metal element inthe metal connecting structure 108 tends to diffuse to the metalreflective layer 106 to cause a decrease in reflectivity of the metalreflective layer 106, and the luminous intensity of the light-emittingdiode is decreased.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a light-emitting device andmanufacturing method thereof. The light-emitting device comprises: ametal connecting structure; a barrier layer on the metal connectingstructure, the barrier layer comprising a first metal multilayer on themetal connecting structure and a second metal multilayer on the firstmetal multilayer; a metal reflective layer on the barrier layer; and alight-emitting stack electrically coupled to the metal reflective layer,wherein the first metal multilayer comprises a first metal layercomprising a first metal material and a second metal layer comprising asecond metal material, the first metal layer closer to the metalconnecting structure than the second metal layer, and the second metalmultilayer comprises a third metal layer comprising a third metalmaterial and a fourth metal layer comprising a fourth metal material,the third metal layer closer to the second metal layer than the fourthmetal layer, and the first metal material is different from the secondmetal material, and the third metal material is different from thefourth metal material.

Furthermore, the present disclosure provides a light-emitting devicecomprising: a metal connecting structure; a barrier layer on the metalconnecting structure, the barrier layer comprising a first metalmultilayer on the metal connecting structure and a second metalmultilayer on the first metal multilayer; a metal reflective layer onthe barrier layer; and a light-emitting stack electrically coupled tothe metal reflective layer, wherein the metal connecting structure andthe metal reflective layer comprise the same metal element, and thebarrier layer comprises metal elements different from a metal element inthe metal reflective layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a conventional light-emitting diode.

FIGS. 2A to 21 show a light-emitting device and manufacturing methodthereof in accordance with a first embodiment of the present disclosure.

FIGS. 3A and 3B show the barrier layers in the first embodiment.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 2 shows a light-emitting device and manufacturing method thereof inaccordance with a first embodiment of the present disclosure. As shownin FIG. 2A, the method for forming the light-emitting device comprisesproviding a growth substrate 201 and forming a light-emitting stack 202on the growth substrate 201. The light-emitting stack 202 comprises asemiconductor stack which comprises from the bottom to the top a firstconductive type semiconductor layer 202 a, a light-emitting layer 202 bon the first conductive type semiconductor layer 202 a, and a secondconductive type semiconductor layer 202 c on the light-emitting layer202 b. The first conductive type semiconductor layer 202 a and thesecond conductive type semiconductor layer 202 c are of differentconductive types. For example, the first conductive type semiconductorlayer 202 a is an n-type semiconductor layer, and the second conductivetype semiconductor layer 202 c is a p-type semiconductor layer. Thefirst conductive type semiconductor layer 202 a, the light-emittinglayer 202 b, and the second conductive type semiconductor layer 202 ccomprise III-V group material, such as AlGaInP series materials.

Next, as shown in FIG. 2B, the method further comprises forming adielectric layer 203 on the light-emitting stack 202. The dielectriclayer 203 has a refractive index smaller than the refractive index ofthe light-emitting stack 202. For example, the dielectric layer 203 canbe silicon oxide (SiO_(x)), magnesium fluoride (MgF₂), and siliconnitride (SiN_(x)). A thickness of the dielectric layer 203 is from about50 nm to 150 nm, and the thickness of the dielectric layer 203 is 100 nmin the present embodiment. Next, as shown in FIG. 2C, the method furthercomprises forming a plurality of through holes 2031 in the dielectriclayer 203 by lithography and etching processes. The through holes 2031penetrate the dielectric layer 203. From the top view, the through holes2031 are substantially circles (not shown) with a diameter D of fromabout 5 μm to 15 μm. The diameter D is about 10 μm in the presentembodiment.

Next, as shown in FIG. 2D, the method further comprises forming a firsttransparent conductive oxide layer 204 on the dielectric layer 203 andfilling into the through holes 2031 so the first transparent conductiveoxide layer 204 and the light-emitting stack 202 form an ohmic contact.A thickness of the first transparent conductive oxide layer 204 is fromabout 25 Å to 100 Å. The thickness of the first transparent conductiveoxide layer 204 is about 75 Å in the present embodiment. Then a secondtransparent conductive oxide layer 205 is formed on the firsttransparent conductive oxide layer 204, wherein the second transparentconductive oxide layer 205 is for providing a current diffusion in alateral direction (a direction perpendicular to the stacking directionof the layers) and comprises a material different from that of the firsttransparent conductive oxide layer 204. A thickness of the secondtransparent conductive oxide layer 205 is from about 0.5 μm to 3 μm, andthe thickness of the second transparent conductive oxide layer 205 is1.0 μm in the present embodiment. It is noted that the secondtransparent conductive oxide layer 205 is much thicker than the firsttransparent conductive oxide layer 204 and the dielectric layer 203, soas shown in the figure, after the formation of the second transparentconductive oxide layer 205, the through holes 2031 are filled up, andthe uneven surface caused by the height difference from the throughholes 2031 becomes a more even surface. The first transparent conductiveoxide layer 204 and the second transparent conductive oxide layer 205comprise a material such as indium tin oxide (ITO), aluminum zinc oxide(AZO), cadmium tin oxide, antimony tin oxide, zinc oxide (ZnO), zinc tinoxide, and indium zinc oxide (IZO). In the present embodiment, the firsttransparent conductive oxide layer 204 comprises indium tin oxide (ITO),and the second transparent conductive oxide layer 205 comprises indiumzinc oxide (IZO).

Next, as shown in FIG. 2E, the method further comprises forming a metalreflective layer 206 on the second transparent conductive oxide layer205, wherein the metal reflective layer 206 comprises a metal materialfor reflecting light emitted by the light-emitting stack 202. In thepresent embodiment, the metal reflective layer 206 has a reflectivitygreater than 90% to light emitted by the light-emitting stack 202. Forexample, the metal reflective layer 206 comprises gold (Au).

Next, as shown in FIG. 2F, the method further comprises forming abarrier layer 207 on the metal reflective layer 206. The barrier layer207 is for preventing metal diffusion between the metal reflective layer206 and the metal connecting structure 208. The metal connectingstructure 208 will be illustrated later. The embodiment of the barrierlayer 207 is shown in FIG. 3A or FIG. 3B, and will be described indetail later. Next, a first bonding layer 2081 is formed on the barrierlayer 207, and a second bonding layer 2082 is formed on the firstbonding layer 2081. Next, as shown in FIG. 2G, a permanent substrate 209is provided, and a third bonding layer 2083 is formed on the permanentsubstrate 209. As shown in FIG. 2H, the third bonding layer 2083 and thesecond bonding layer 2082 are bonded together, and the growth substrate201 is removed after bonding. The first bonding layer 2081, the secondbonding layer 2082, and the third bonding layer 2083 form the metalconnecting structure 208. The metal connecting structure 208 comprises alow-temperature fusion material with a melting point lower than or equalto 300° C. For example, the low-temperature fusion material comprisesindium (In) or tin (Sn). In the present embodiment, the low-temperaturefusion material comprises indium (In). For example, when the firstbonding layer 2081 comprises gold (Au), the second bonding layer 2082comprises indium (In), and the third bonding layer 2083 comprises gold(Au), the first bonding layer 2081, the second bonding layer 2082, andthe third bonding layer 2083 may form an alloy and be bonded together toform the metal connecting structure 208 at a low temperature, such as atemperature lower than or equal to 300° C., because of the eutecticeffect. The metal connecting structure 208 comprises an alloy of indium(In) and gold (Au). In another embodiment, the second bonding layer 2082is formed on the first bonding layer 2081, and is bonded to the thirdmetal bonding layer 2083 on the permanent substrate 209 to form themetal connecting structure 208.

Then, as shown in FIG. 2I, the method further comprises forming a firstelectrode 210E1 and its extending electrode 210E1′ on the light-emittingstack 202. Then, a peripheral part of the light-emitting stack 202 isremoved to expose a part of the dielectric layer 203 by lithography andetching processes. A roughening process may be performed optionally onthe surface of the light-emitting stack 202 to form a roughenedstructure 212 r on the first conductive type semiconductor layer 202 a.A protective layer 211 is then formed on the light-emitting stack 202and on the exposed part of the dielectric layer 203, wherein theprotective layer 211 does not cover the first electrode 210E1 and itsextending electrode 210E1′. Finally, a second electrode 210E2 is formedon the permanent substrate 209.

FIG. 3A is used for illustrating the barrier layer 207 in the aboveembodiment. FIG. 3A illustrates the barrier layer 207 in FIG. 2I, soFIG. 3A is a reference to FIG. 2I at the same time. As mentionedpreviously, the barrier layer 207 is disposed between the metalreflective layer 206 and the metal connecting structure 208 to preventthe metal diffusion between them. The barrier layer 207 in the presentembodiment comprises a first metal multilayer 2071 on the metalconnecting structure 208 and a second metal multilayer 2072 on the firstmetal multilayer 2071. The first metal multilayer 2071 comprises a firstmetal layer 2071 a comprising a first metal material and a second metallayer 2071 b comprising a second metal material. The first metal layer2071 a is closer to the metal connecting structure 208 than the secondmetal layer 2071 b is. The second metal multilayer 2072 comprises athird metal layer 2072 a comprising a third metal material and a fourthmetal layer 2072 b comprising a fourth metal material. The third metallayer 2072 a is closer to the metal connecting structure 208 than thefourth metal layer 2071 b is. Regarding the selection of the materials,the first metal material is different from the second metal material,the third metal material is different from the fourth metal material,and the selection of these metal materials leads to the barrier layer207 comprises metal elements which are different from a metal element inthe metal reflective layer 206. In the present embodiment, the firstmetal layer 2071 a and the third metal layer 2072 a comprise platinum(Pt), and the second metal layer 2071 b and the fourth metal layer 2072b comprise titanium (Ti). The platinum (Pt) in the first metal layer2071 a and the third metal layer 2072 a is mainly a material to preventmetal diffusion between the metal reflective layer 206 and the metalconnecting structure 208. The titanium (Ti) in the second metal layer2071 b and the fourth metal layer 2072 b is to increase adhesion force.In particular, the titanium (Ti) in the fourth metal layer 2072 b whichconnects with the metal reflective layer 206 provides a good adhesionbetween the barrier layer 207 and the metal reflective layer 206. Thatis, a good material selection or arrangement for these metal materialsleads to a result that the adhesion between the fourth metal layer 2072b and the metal reflective layer 206 is stronger than the adhesionbetween the third metal layer 2072 a and the metal reflective layer 206.Thus the adhesion between the third metal layer 2072 a and the metalreflective layer 206 is reinforced. Regarding the thicknesses of thelayers, the thicknesses of the first metal layer 2071 a and the thirdmetal layer 2072 a are from about 100 Å to 500 Å and the thicknesses ofthe second metal layer 2071 b and the fourth metal layer 2072 b are fromabout 200 Å to 800 Å. In the present embodiment, the thicknesses of thefirst metal layer 2071 a and the third metal layer 2072 a are from about200 Å to 800 Å and the thicknesses of the second metal layer 2071 b andthe fourth metal layer 2072 b are from about 100 Å to 500 Å. The aboveranges of the thicknesses makes the first metal multilayer 2071 and thesecond metal multilayer 2072 be able to prevent metal diffusion betweenthe metal reflective layer 206 and the metal connecting structure 208and a stress problem is not occurred because of thick thicknesses. Thestress problem may have an impact on the following bonding process ofthe bonding layers of the metal connecting structure 208.

According to the final structure in FIG. 2I with reference to FIG. 3A,the light-emitting device in the first embodiment of the presentapplication comprises the metal connecting structure 208; the barrierlayer 207 on the metal connecting structure 208, wherein the barrierlayer 207 comprises the first metal multilayer 2071 on the metalconnecting structure 208 and the second metal multilayer 2072 on thefirst metal multilayer 2071; the metal reflective layer 206 on thebarrier layer 207; and the light-emitting stack 202 electrically coupledto the metal reflective layer 206. The first metal multilayer 2071comprises the first metal layer 2071 a comprising the first metalmaterial like platinum (Pt) and the second metal layer 2071 b comprisingthe second metal material like titanium (Ti), and the first metal layer2071 a is closer to the metal connecting structure 208 than the secondmetal layer 2071 b. The second metal multilayer 2072 comprises the thirdmetal layer 2072 a comprising a third metal material like platinum (Pt)and the fourth metal layer 2072 b comprising a fourth metal materiallike titanium (Ti), and the third metal layer 2072 a is closer to thesecond metal layer 2071 b than the fourth metal layer 2072 b.Furthermore, as mentioned above, the metal connecting structure 208comprises an alloy of indium (In) and gold (Au), and the metalreflective layer 206 comprises gold (Au) in the present embodiment. Thatis, the metal connecting structure 208 and the metal reflective layer206 comprise the same metal element gold (Au). As mentioned in thebackground, because the same metal element exists on both sides of thebarrier layer 207, other elements (e.g. indium (In) in the presentembodiment) in the alloy of the metal connecting structure 208 areparticularly easy to be combined on both sides of the barrier layer 207.Therefore, if a thin barrier layer is used, it is unable to effectivelyprevent metal diffusion of indium (In) between the metal reflectivelayer 206 and the metal connecting structure 208. An element analysis byEDS line scan is done for a structure shown in FIG. 2I with the barrierlayer 207 changed to a thin barrier layer, for example, a single layerof platinum (Pt) with a thickness of 500 Å. The analysis result showsthat indium (In) content in the metal reflective layer 206 and indium(In) content in the metal connecting structure 208 are close, whereinboth are from about 5 to 10 A.U. (Arbitrary Unit), with an average valueof about 7.5 A.U. This result proves that using the thin barrier layeris unable to effectively prevent metal diffusion of indium (In) betweenthe metal reflective layer 206 and the metal connecting structure 208.In contrast, when the aforementioned structure in FIG. 3A is used as thebarrier layer 207, because the barrier layer 207 in FIG. 3A comprises aplurality of multilayer structures comprising the first metal multilayer2071 and the second metal multilayer 2072, and the barrier layer 207comprises metal elements different from a metal element in the metalreflective layer 206. The barrier layer 207 is able to effectivelyprevent metal diffusion of indium (In) between the metal reflectivelayer 206 and the metal connecting structure 208. In addition, comparedwith the method which tries to raise the barrier layer's ability toprevent metal diffusion by simply increasing the thickness, the barrierlayer 207 in FIG. 3A avoids stress problems arisen from increasing thethickness of the barrier layer. Therefore, when an element analysis isdone for the structure by EDS line scan, the analysis result shows thatindium (In) content in the metal reflective layer 206 significantlydecreases and is different from indium (In) content in the metalconnecting structure 208. The indium (In) content in the metalreflective layer 206 is substantially the same as indium (In) content inthe light-emitting stack 202, and both are less than about 5 A.U.(Arbitrary Unit), with an average value of about only 2 A.U. That is,indium (In) content in the metal reflective layer 206 (an average valueof about 2 A.U.) is less than a half of indium (In) content in the metalconnecting structure 208 (an average value of about 7.5 A.U.). Thisresult proves that using the barrier layer in the present embodiment isable to effectively prevent metal diffusion of indium (In) between themetal reflective layer 206 and the metal connecting structure 208.

It is noted that the illustration for the barrier layer 207 in FIG. 3Ais based on FIG. 2I, i.e., the final structure of the light-emittingdevice. However, as mentioned previously, the structure in FIG. 2I isformed by making the growth substrate 201 upside down to bond with thepermanent substrate 209. Therefore, in regard to the forming method,taking an intermediate process figure like FIG. 2F as an example, thefourth metal layer 2072 b, the third metal layer 2072 a, the secondmetal layer 2071 b, and the first metal layer 2071 a are sequentiallyformed on the metal reflective layer 206.

FIG. 3B shows another embodiment of the barrier layer of the presentapplication. FIG. 3B is a modification of FIG. 3A, and FIG. 3B alsoillustrates the barrier layer 207 in FIG. 2I. Therefore, FIG. 3B is areference to FIG. 2I at the same time. Similarly, the barrier layer 207is disposed between the metal reflective layer 206 and the metalconnecting structure 208 to prevent metal diffusion between them. Thebarrier layer 207 in the present embodiment is substantially the same asthe barrier layer 207 in FIG. 3A, but the material of the first metallayer 2071 a′ in the present embodiment comprises nickel (Ni), and ananti-oxidation layer 207 i is added between the first metal multilayer2071 and the second metal multilayer 2072 to prevent the oxidation ofthe second metal multilayer 2072 in the process, wherein theanti-oxidation layer 207 i comprises gold (Au), with a thickness ofabout 3000 Å to 7000 Å. Other details like the material or thickness areas those illustrated previously in FIG. 3A, and are not described againhere. Similarly, in regard to the forming method, taking an intermediateprocess figure like FIG. 2F as an example, the fourth metal layer 2072b, the third metal layer 2072 a, the anti-oxidation layer 207 i, thesecond metal layer 2071 b, and the first metal layer 2071 a aresequentially formed on the metal reflective layer 206. Theanti-oxidation layer 207 i can prevent the oxidation of the second metalmultilayer 2072 in the process, when the second metal multilayer 2072and the first metal multilayer 2071 are not formed continuously in thesame equipment.

The above-mentioned embodiments are only examples to illustrate thetheory of the present invention and its effect, rather than be used tolimit the present application. Other alternatives and modifications maybe made by a person of ordinary skill in the art of the presentapplication without departing from the spirit and scope of theapplication, and are within the scope of the present application.

What is claimed is:
 1. A light-emitting device comprising: a substrate;a barrier layer on the substrate, the barrier layer comprising a firstmetal multilayer and a second metal multilayer directly on the firstmetal multilayer; a metal connecting structure located between thesubstrate and the barrier layer; a metal reflective layer on the barrierlayer; and a light-emitting stack electrically coupled to the metalreflective layer, wherein the barrier layer is between thelight-emitting stack and the substrate; wherein the first metalmultilayer comprises a first metal layer comprising a first metalelement and a second metal layer comprising a second metal element, andthe first metal layer is closer to the metal connecting structure thanthe second metal layer is, and the second metal multilayer comprises athird metal layer comprising a third metal element and a fourth metallayer comprising a fourth metal element, and the third metal layer iscloser to the second metal layer than the fourth metal layer is, and thefirst metal element is different from the second metal element, and thethird metal element is different from the fourth metal element.
 2. Thelight-emitting device of claim 1, wherein the metal connecting structureand the metal reflective layer comprise the same metal element.
 3. Thelight-emitting device of claim 1, wherein the first metal layer consistsof the first metal element and the third metal layer consists of thethird metal element.
 4. The light-emitting device of claim 1, wherein athickness of the first metal layer is different from that of the secondmetal layer.
 5. The light-emitting device of claim 4, wherein thethickness of the first metal layer is larger than that of the secondmetal layer.
 6. The light-emitting device of claim 1, wherein thelight-emitting stack comprises a surface away from the metal connectingstructure, and the surface comprises a first part and a second parthaving a roughness larger than that of the first part.
 7. Thelight-emitting device of claim 6, further comprising an electrodelocated on the first part.
 8. The light-emitting device of claim 1,further comprising a dielectric layer between the barrier layer and thelight-emitting stack.
 9. The light-emitting device of claim 8, whereinthe dielectric layer comprises a plurality of through holes whichpenetrate the dielectric layer.
 10. The light-emitting device of claim8, wherein the dielectric layer comprises silicon oxide (SiO_(x)),magnesium fluoride (MgF₂), or silicon nitride (SiN_(x)).
 11. Thelight-emitting device of claim 1, wherein the metal reflective layercomprises a fifth metal element different from the first metal elementand the second metal element, or the metal connecting structurecomprises a sixth metal element different from the first metal elementand the second metal element.
 12. The light-emitting device of claim 1,wherein the metal connecting structure comprises indium, gold or both.13. The light-emitting device of claim 1, wherein the metal reflectivelayer comprises gold.
 14. The light-emitting device of claim 1, whereinthe first metal layer comprises the same material as the third metallayer.
 15. The light-emitting device of claim 3, wherein the first metalelement is the same as the third metal element.
 16. The light-emittingdevice of claim 1, wherein the first metal element comprises nickel orplatinum.
 17. The light-emitting device of claim 1, wherein the secondmetal layer comprises the same material as the fourth metal layer. 18.The light-emitting device of claim 1, wherein the second metal layerconsists of the second metal element and the fourth metal layer consistsof the fourth metal element.
 19. The light-emitting device of claim 18,wherein the second metal element is the same as the fourth metalelement.
 20. The light-emitting device of claim 1, wherein the secondmetal element and the fourth metal element comprise titanium.